This invention relates to a voltage to pulse-width conversion circuit for adjusting the brightness of an electronic indicating device such as a fluorescent character display tube or the like.
Some automobiles are provided with an electronic indicating panel on a dashboard which is controlled by a driving circuit and which may be used, for example, for an audio system, etc.
It is desirable to dim suitably the brightness of the electronic indicating panel during a night drive, because the glare of the element fatigues a driver.
For this reason, a voltage to pulse-width conversion circuit has been used to control the pulse-width of a drive signal from the driving circuit to the segment electrode of an indicating panel for adjusting the brightness of the indicating panel.
The known voltage to pulse-width conversion circuits can be roughly divided into analog type and digital type systems. Their structures will now be described with reference to the accompanying drawings.
FIG. 2 shows one of conventional analog type voltage to pulse-width conversion circuits which comprises a voltage comparator 20 having a positive input for receiving a pulse width control signal S1 through an input terminal 1 and an output 2 for producing a pulse width modulation signal (hereinafter referred to as a PWM signal) S2, and a CR oscillation circuit 10. A CR oscillation circuit 10 has an output terminal connected to the negative input of voltage comparator 20. An output of the voltage comparator 20 is connected to the output terminal 2.
The CR oscillation circuit 10 produces sawtooth waveform signals S10. This circuit consists of a voltage comparator 11 having an NPN transistor in a collector open state, a capacitor 12, and five resistors 13-17. The voltage comparators 11 and 20 operate on the positive voltage V10 of a current source used for an electronic indicator device (not shown in the figure). A signal S11a divided by the resistors 13 and 14 is applied to the positive input of the voltage comparator 11. The voltage comparator 11 has a negative input for receiving a signal S11b which is supplied through resistor 17.
The input terminal 1 is connected to an input circuit which supplies a PWM input signal S1. This input circuit has a switch 30 for illumination of the interior of an automobile during a night drive. The switch 30 has one terminal connected to a positive voltage V30 of a 13.8 V battery and the other terminal connected to a variable resistor 31 which is normally adjusted by a driver. The variable resistor 31 supplies a divided voltage across the resistors 32 and 33, an intermediate node between the registers 32 and 33 is connected to the input terminal 1.
FIG. 3 shows signal waveforms generated in the circuit of FIG. 2. FIG. 4 is a chart which illustrates input-output characteristics of the circuit of FIG. 2. Operation of the circuit of FIG. 2 will now be described with reference to FIGS. 3 and 4.
When the voltage V10 is applied to the CR oscillation circuit 10 and the voltage comparator 20 of FIG. 2, the CR oscillation circuit 10 begins to oscillate and normally generates on its output an oscillation signal S10 having a frequency of about 128 Hz. This signal S10 is fed to the negative of voltage comparator 20. The frequency and waveform of oscillation signal S10 are determined by the time constant of the capacitor 12 and the resistors 13-17. More specifically, when capacitor 12 is charged through the resistors, the potential of an input signal S11b on the negative input of comparator 11 is increased. At this moment, the potential of input signal S11a entering the positive-input of voltage comparator 11 also is slightly increased under the effect of a feed back resistor 15 based on a reference potential divided by resistors 13 and 14. A feed back resistor 17 has a much lower value of resistance than those of the other resistors 13-16. When the potential of an input signal S11b on the negative input of the voltage comparator 11 becomes higher than that of the input signal S11a, the output signal S10 of the voltage comparator 11 is set to a low level (hereinafter designated merely by the letter "L"). Therefore, the charge voltage on capacitor 12 is quickly discharged to the ground (=OV) through resistor 17. At the same time, the input signal S11a also quickly decreases its potential. The amount of such a change in the potential is determined by values of the resistors 13 to 17. When the potential of the input signal S11b falls below that of the input signal S11a, the voltage comparator 11 is set OFF, and until the potential of the input signal S11b becomes higher than that of the input signal S11a, the capacitor 12 will be charged through the resistors. Such an action will sustain operation of the CR oscillation circuit 10.
For example, when the illumination switch 30 is in the ON position during a night drive, the battery voltage V30 is supplied across the variable resistor 31. A voltage divided by the variable resistor 31 is subdivided by the voltage dividing resistors 32 and 33. The voltage at the point between resistors 32 and 33 is fed to the input terminal 1 in the form of a PWM input signal S1 supplied to the positive input of the voltage comparator 20. When the variable resistor 31 is manually adjusted, the level of the PWM input signal S1 varies as shown in FIG. 3 as S1-1 and S1-2. The voltage comparator 20 compares the level of the output signal S10 with that of the PWM input signal S1, performs a voltage to pulse-width conversion, and supplies a PWM output signal S2 to the output terminal 2. The PWM output signal S2 first falls into a high level (hereinafter designated merely by the letter "H") at a range of S10 &lt; S1 and then becomes "L" at a range of S10 &gt; S1.
The input-output characteristics of PWM output signal S2 versus PWM input signal S1 as shown in FIG. 4 are determined by the output waveform of the CR oscillation circuit 10. However, there is a limit to the obtainable accuracy (closeness to the ideal characteristic) in the above described method (of selecting the resistors 13 to 17 and capacitor 12).
FIG. 5 illustrates an example of a conventional digital type voltage to pulse-width conversion circuit.
This voltage to pulse-width conversion circuit has an analog/digital converter (hereinafter referred to as an A/D converter). The circuit has an input terminal 40 for receiving a PWM input signal S1, an input terminal 41 for receiving a reference frequency signal S0, and an output terminal 42 for generating a PWM output signal S2. Installed in series between the input terminal 40 and the output terminal 42 are a ten-bit A/D converter 43, a (1024 word x ten-bit) read-only memory (hereinafter referred to as ROM) 44, and a ten-bit PWM generation circuit 45. The input terminal 41 is also connected to an input of a timing generation circuit 46. An output of the circuit 46 is connected to the A/D converter 43, the ROM 44, and the PWM generation circuit 45.
Operation of this system will now be described.
When a reference frequency signal S0 is supplied to the input terminal 40, the timing generation circuit 46 generates a timing signal to be supplied to the A/D converter 43, the ROM 44, and the PWM generation circuit 45. The A/D converter 43 converts the PWM input signal S1 into a digital signal to be transmitted to the ROM 44. The ROM 44 reads out the data corresponding to the digital signal, and the PWM generation circuit 45 generates a PWM output signal S2 corresponding to the received data to the output terminal 42.
Nevertheless, the circuits described above have the following disadvantages:
(I) In the conventional analog type voltage to pulse-width conversion circuit, because the CR oscillation circuit 10 must generate a low frequency signal of 128 Hz, each of the resistors 13-17 requires a very high resistance value i.e., of about several hundred kOhm. Therefore, it is very difficult to integrate the CR oscillation circuit 10 in a semiconductor IC chip. The variations in resistance values of the resistors 13-17 due to process conditions affect the output oscillation waveform and decrease the frequency stability of the PWM output signal S2. The use of external resistors 13-17 increases the number of IC lead terminals. It will not be practical to make a voltage to pulse-width conversion circuit by using discrete components, because there is generally not enough mounting space on the back side of the indicating panel of a car radio (tuner).
As the characteristics of the PWM input signal S1 and the PWM output signal S2 are determined by the oscillation signal S10, it is not possible to vary arbitrarily the waveform of PWM output signal S2. The capacitance deviation of capacitor 12 which determines the waveform of oscillation signal S10 affects the accuracy of oscillation signal S10.
(II) The digital type voltage to pulse-width conversion circuit shown in FIG. 5 is large in circuit scale. This leads to an increase in chip size as well as in cost. In a real construction, it is difficult to integrate the conversion circuit with a drive circuit and other circuits into one chip.